This invention relates to an automatic brightness limiter circuit (ABL) for television sets and offers a highly suitable ABL circuit for connecting a personal computer to the television set and superimposing data from the personal computer onto the screen of the television set and displaying both simultaneously.
It is well known that, in normal color television sets, the upper limit of the beam current in the cathode ray tube is held to a certain value using an ABL circuit for the purpose of protection against damaging the horizontal output transistor and the horizontal output transformer due to high beam current.
When displaying television broadcasts and data displays of digital signals from a personal computer simultaneously, the signals composing the computer display are discontinuous "1" or "0" digital signals whereas the TV display is composed of picture signals which change continuously according to time, excluding blanking periods. It is desirable in composite displays formed of these two signals that the TV picture signals be erased or the contrast lowered for ease of viewing during the period that the digital signals exist; the television broadcast pictures in the composite display thereby forming a data background display for the digital signals.
In a television set receiving the foregoing composite display, the beam current I.sub.K of the cathode ray tube will be the sum of the broadcast display component I.sub.L and display component I.sub.D of the digital signal. In other words, it may be expressed as follows. EQU I.sub.K =I.sub.L +I.sub.D
Here, if we feed back the detected beam current value I.sub.K to the brightness control circuits of both the broadcast display and the digital signal display as in conventional ABL circuits, it may be expressed by the following. EQU I.sub.L =f(I.sub.K) EQU I.sub.D =g(I.sub.K)
Components I.sub.L and I.sub.D vary as functions of the overall beam current I.sub.K. That is, as the contrast, or brightness (as the digital signals are composed of values of "1" or "0", contrast and brightness will be the same), changes according to the broadcast display forming the background, the display will become difficult to view, the characters of the digital signal display being particularly difficult to read especially when the brightness of the broadcast display changes greatly or when it changes rapidly timewise.